1. Field of Invention
There are numerous industrial processes which require the transport of solids through ducts using a carrier gas steam. Examples are the food industry, the pharmaceutical industry and, especially, combustion processes using solid fuel, such as power plants, cement factories and blast furnaces.
Although for gases and liquids there has traditionally been a wide variety of equipment for continuous measurement of the mass flow through ducts, in the case of pneumatically transported solids, it is only in recent years that promising technologies have been developed for determining this parameter.
The importance of measuring the mass flow of solids depends on the particular process involved. For coal-fired power stations this measurement is of vital importance from the point of view of the performance and operation of the plant and the emission of pollutants.
Most coal-fired power stations use direct fuel feed systems. In these systems, the coal or other solid fuel is pulverised in several mills, where a stream of hot air dries the coal and carries it through several ducts to the burners located in the boiler furnace. The number of burners fed by each mill varies from one installation to another, but there are generally 4-6 of them. The transport ducts from the mills to the burners are, in general, of different lengths and layouts, and with different types and numbers of bends and changes of direction. This difference in hydraulic resistance between different ducts, and possible asymmetries in the flow inside the mills or the classifiers causes significant imbalances in the supply of coal and air to each burner.
These imbalances have a negative influence on performance and on the generation of pollutants in this type of installation, in such a way that the existence of critical areas with an inappropriate air/coal ratio causes penalties with respect to these extremely important parameters. These imbalances can also generate areas of high reduction which cause a drop in the fusion point of the ash, and, consequently, increase the propensity of ash deposition on the boiler pipes (slagging). This causes significant losses due to heat transfer, and at times causes problems which make it necessary to shut down the plant, with the consequent loss of production.
A common practice in coal-fired power stations to reduce these imbalances is to add orifices of different diameters into the ducts to the burners, causing different pressure drops in order to equalise the hydraulic resistance of the different ducts.
The flow is balanced in each duct by means of measurements taken during tests using clean air, with no coal. Due to the peculiarities of the flow of biphasic mixtures, this balancing with clean air does not guarantee a balanced distribution of coal during normal operation of the mill. In addition, the imbalances in the flow of coal between the different ducts of a mill varies, in magnitude and direction, with mill operation, and so the above adjustment has a limited efficiency.
The environmental limitations of recent years have encouraged the use of low NOx burners which minimise the generation of this pollutant by adjusting the stoichiometry of the flame. The optimum operation of this type of burner is conditioned by a tight control of the air/coal ratio.
All of the foregoing underlines the need for providing these installations with control loops to regulate the supply of coal to the boiler. These control loops should be based on adjustable mechanisms, such as dampers, directional vanes, etc., as well as on reliable measurements of the flow of coal to each of the burners.
2. Description of Related Prior Art
At present there exists a wide range of equipment for measuring the flow of pneumatically transported solids. This equipment falls into two large groups: extractive and non-extractive equipment, depending on whether samples are taken from the ducts.
Extractive equipment aspirates a sample of the solids carried in each duct. This sample, after being weighed and analysed in the laboratory, makes it possible to obtain the mass flow and the particle size distribution.
This equipment is based mainly on international standards developed for the sampling of pulverised coal. These standards, ASTM D197-87, ISO 9931 and the ESI Industry Standard, specify a method and the basic components of pulverised coal sampling equipment in circular ducts, with the ultimate aim of determining the flow of coal and the particle size distribution.
The ASTM D197-87 standard establishes the use of manual equipment, using a probe equipped with a single nozzle which is taken to different locations on two diameters at 90° to each other in a transversal section of the duct. The sample must be aspirated at the same velocity as the carrier air stream (isokinetic velocity). This aspiration velocity must be adjusted in order to obtain a true reading, since a lower velocity would give a sample with a lack of fines and a higher velocity would overestimate these.
The most commonly used method for sampling pneumatically transported solid particles is that specified by the ISO 9931 standard. This standard has been used as a reference by several manufacturers of equipment, such as M&W Asketeknik, GEE-EER, INERCO and Mission Instruments. These are all manual, with a rotating probe and four aspiration nozzles, each one of which sweeps the same area of the transversal section of the duct as it turns. The sample is extracted isokinetically by means of adjusting the suction provided by an ejector and is separated from the air flow by a cyclone.
This last method presents, in comparison to the previous one, a significant improvement as regards the representativity of the sample, and is less sensitive to the effects of particle segregation produced by disturbances in the flow.
Equipment based on the ESI Industry Standard uses null pressure probes which are manually positioned at different places on the transversal section of the duct, aspirating at each point at isokinetic velocity. Examples of commercial equipment using this method are the SMG-10 and AKOMA systems. The advantage of this method, in comparison to the others, is that it aspirates isokinetically at all points of the section, and its main disadvantage is the length of time needed for the sampling.
Other equipment, based on the methods described above, has also been developed to automate sampling. One example of this type of equipment is the APFS developed by M&W Asketeknik (patent W09810266). This equipment is based on the ISO 9931 standard, with regard to the shape of the probe and aspiration nozzles, as well as the component parts and the operating principle. The innovation it includes is the installation in each duct of probes which can be inserted and retracted from the duct automatically using a rack and pinion type mechanism actuated by an electric motor.
As an advanced version of the above equipment, M&W Asketeknik has developed the ACFM-2100, which, using the same probe and the same actuating mechanism, incorporates in a control unit a cyclone for separating particles, connected to a continuous system for inferring the amount of sample extracted during each sampling. This system consists of a cylindrical chamber which during sampling is filled to a certain level. An optical sensor detects the level of particles in the chamber and therefore the volume of the sample extracted. The flow of solids in the duct is inferred from the apparent density of the particles.
The disadvantage of this system is that it produces results by indirect measurement of the weight of the particles by determining the volume of the sample. This may give rise to significant errors in the case of changes in the density of the solid or distribution of the size of particles, requiring continuous adjustment of the equipment to guarantee the quality of the measurement.
The other group of equipment for the measurement of the mass flow of particles is made up of non-extractive systems, which have developed greatly in recent years. These systems are based on the measurement of some physical property in the biphasic flow related to the concentration of the particles. In addition to methods based on electrostatic, acoustic and mechanical methods, all the regions of the electromagnetic spectrum, from gamma rays to microwaves, have been used for the development of these sensors.
These measurement systems have the disadvantage of not being very precise and accurate, due to the fact that they are sensitive to parameters such as the humidity content of the mixture, the size of particles and the phenomena of segregation in the ducts. In addition, they require periodic calibration using comparative measurements carried out using extractive methods, and do not allow for the obtention of a sample for later laboratory analysis.
Of all the documents analysed, those which most resemble the device in question are systems for weighing bulk materials, either for continuous flow measurement or for packaging or bagging. Examples are U.S. Pat. No. 4,513,830 (Teccon) and U.S. Pat. No. 5,121,638 (Buehler). However, these devices, in addition to having other aims, lack, among others, the characteristics related to the particle expulsion system.
Within the specific field of particle transport in a gas stream, there is U.S. Pat. No. 4,758,118 (Rachner & Schott), although the aim of the procedure and the device described is not so much the measurement of the flow of an already established stream, but the previous stage of introduction of a measured amount of particle material in a gas stream to make up a particular mass flow. For this purpose, the weight of the pulverised material is measured using a container for weighing which uses electrical load cells on which that container rests (column 5, lines 8 to 17). The description of the weighing container, however, is not given in more detail, and so it can be considered that it does not affect the novelty of the device as described in claims 1 to 7.
U.S. Pat. No. 4,490,077 (Nippon Kohan KK/Sankyo Dengyo KK), in the same sector, presents a similar case, where load cells are also used for continuous measurement of particles in a container. Here, again, the brief description of the weighing device allows us to conclude that it is not relevant for the application analysed. Other patents have been recovered with similar aims (dosed feed of particles to gas streams by weighing), but none of these concentrates on the weighing device. In some of these, indeed, such as U.S. Pat. No. 4,669,921 (Charbonnages de France), it is specified that the gravimetric measuring device used is one available on the market (column 3, lines 11 to 17).
Another document related to this subject is U.S. Pat. No. 4,838,738 (Shell Oil Co.), where the mass flow of a mixture of solid particles and gas supplied to a reactor is controlled by determining the weight of the mixture in a container. In this patent also, although a weighing device is used, the general layout is different.
All of this means that at present none of these technologies has reached a sufficiently robust state for large-scale industrial implementation.